Lateral silicon-on-insulator bipolar junction transistor radiation dosimeter

ABSTRACT

A radiation dosimeter includes a semiconductor substrate and a buried insulator layer disposed on the semiconductor substrate. The buried insulator layer has a plurality of charge traps. A semiconductor layer is disposed on the buried insulator layer. The semiconductor layer has an emitter, an intrinsic base, and a collector laterally arranged with respect to one another. In response to radiation exposure by the radiation dosimeter, positive charges are trapped in the plurality of charge traps in the buried insulator layer, the amount of positive charge trapped being used to determine the amount of radiation exposure. A method for radiation dosimetry includes providing a radiation dosimeter, where the radiation dosimeter includes a lateral silicon-on-insulator bipolar junction transistor having a buried insulator layer; exposing the radiation dosimeter to ionizing radiation; determining a change in one of the collector current and current gain of the radiation dosimeter; and determining an amount of the radiation dose based on the change in one of the collector current and current gain.

FIELD

This disclosure relates generally to the field of radiation monitoringand dosimetry.

BACKGROUND

At present, several types of personal radiation monitors are used forrecording exposure to X-rays, γ-rays, β-radiation, and other kinds ofionizing radiation. The most commonly used are ionization detectors,Geiger counters, and thermoluminescent dosimeters (TLDs).

Ionization detectors and Geiger counters can record and display the doserate (for example, in mrad/hr), as well as the integrated dose (forexample, in rads) in real time. Alarm set points can be programmed foreach of these types of monitors with respect to either dose rate orintegrated dose. Both types can communicate to personal computers fordata logging or firmware updates, and both may be relatively expensive.

TLDs enable radiation dose to be determined based on the emission ofphotons which occurs when the dosimeter is heated. TLDs are relativelyinexpensive, but are generally processed (that is, read out) inspecially designed “readers” after a period of exposure time, typicallyafter being deployed or worn for a period of time between one and threemonths. As such, they provide information only on the integrated dosefor the period of time in question, and a dose rate averaged across thatsame period of time. Their disadvantage, of course, is that anyradiation exposure is only learned after the fact, that is, no real timereadout is available.

Following the events of Sep. 11, 2001, concern about the potentialdetonation of radiological-dispersal or “dirty” bombs in majormetropolitan areas has grown. To counter this perceived threat, manylarge-scale detectors have been deployed in ports to monitor shippingcontainers, at airports, and at other points of entry into the country.In addition, many radiation detectors have been installed on top of andinside buildings in major cities, so that radiation exposure levels maybe determined at any time.

In the event of a radiological incident with a serious risk of aradiation leak, such as the detonation of a “dirty” bomb or a crisis ata nuclear reactor site, such as that caused by the earthquake andtsunami in Fukushima, Japan in March 2011, first responders will need toknow an individual's exposure level quickly, so that effective triagemay be established. Radiation monitors within buildings could be used toassess how a radiation plume is spreading, but the critical informationneeded to treat exposed patients, or to assuage the fears of thosepresumed to have been exposed, would be a knowledge of the radiationdose to which individual had actually been exposed.

SUMMARY

In one aspect of the present invention, a radiation dosimeter comprisesa semiconductor substrate and a buried insulator layer disposed on thesemiconductor substrate. The buried insulator layer comprises aplurality of charge traps. A semiconductor layer is disposed on theburied insulator layer. The semiconductor layer has an emitter, anintrinsic base, and a collector laterally arranged with respect to oneanother. When the radiation dosimeter is exposed to radiation, positivecharge is trapped in the plurality of charge traps in the buriedinsulator layer, the amount of positive charge trapped being used todetermine the amount of radiation exposure.

In another aspect of the present invention, a method for radiationdosimetry includes providing a radiation dosimeter, said radiationdosimeter including a lateral silicon-on-insulator (SOI) bipolarjunction transistor (BJT) having a buried insulator layer; exposing theradiation dosimeter to ionizing radiation; determining a change in oneof the collector current and current gain of the radiation dosimeter;and determining an amount of the radiation dose based on the change inone of the collector current and current gain.

Additional features are realized through the techniques of the presentexemplary embodiment. Other embodiments are described in detail hereinand are considered a part of what is claimed. For a better understandingof the features of the exemplary embodiment, the reader is directed tothe description and to the drawings which follow.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Referring now to the drawings wherein like elements are numbered alikein the several figures.

FIG. 1 illustrates a cross section of an embodiment of a lateral SOI BJTradiation dosimeter.

FIG. 2 is a plot of collector current against base/emitter junctionvoltage for several values of a back gate voltage.

FIG. 3 is a plot of base current against base/emitter junction voltagefor several values of a back gate voltage.

FIG. 4 illustrates a cross section of an embodiment of a lateral SOI BJTradiation dosimeter while being exposed to radiation.

FIG. 5 illustrates a method for radiation dosimetry using a lateral SOIBJT radiation dosimeter.

DETAILED DESCRIPTION

In the present disclosure, then, a radiation dosimeter is fabricated ona lateral SOI bipolar junction transistor capable of detecting doses ofvarious kinds of ionizing radiation. The radiation dosimeter exhibitslong-term charge retention that enables long-term tracking of radiationdose. The present device is a thin-base lateral bipolar junctiontransistor (BJT) fabricated on a SOI wafer using a CMOS (complementarymetal-oxide-semiconductor) compatible process.

A lateral SOI BJT radiation dosimeter may be relatively small andinexpensive, and may be embedded in automobiles, buildings, air filters,or portable electronic devices such as computers, cell phones, musicplayers, PDAs or GPSs, or in other items, including, but not limited to,passports, credit cards, or driver licenses. The device may be incommunication with a radiofrequency (RF) tag that may communicate aradiation dosage experienced by the lateral SOI BJT radiation dosimeterto an RF tag reader. Integrated radiation dose information may bedetermined from the radiation dosimeter so that treatment decisions maybe made quickly in an emergency situation. The radiation dosimeter mayalso be implanted into the body of a patient undergoing radiationtherapy, in order to determine radiation dosage to a tumor, or an amountof radiation received during medical imaging. Real-time radiation doseinformation may be gathered from the implanted radiation dosimeter toconfirm that a proper dose of radiation is delivered to a patient. Theradiation dosimeter may be used in conjunction with a relatively smallbattery or precharged capacitor. The radiation dosimeter may also beelectrically connected with one or more inductors, and be used inconjunction with an LC circuit.

FIG. 1 is a cross-sectional view of an embodiment of the lateral SOI BJTradiation dosimeter 100 of the present invention. The radiationdosimeter comprises an intrinsic base 106, an emitter 102, and acollector 103, disposed in a lateral relationship with respect to oneanother. The radiation dosimeter also includes a buried insulator layer104, and a semiconductor substrate 105. An extrinsic base 101 provideselectrical connection to the intrinsic base 106 and functions as thebase terminal of the BJT. Extrinsic base 101 is doped with an acceptormaterial, and intrinsic base 106 between emitter 102 and collector 103is doped with an acceptor material to a lesser extent than extrinsicbase 101. Emitter 102 and collector 103 are doped with a donor materialto approximately the same extent as extrinsic base 101 is doped with anacceptor material. Optionally, a lightly doped collector region can beinserted between the intrinsic base region and heavily doped collectorregion. Spacer material 108 surrounds extrinsic base 101. Optionally,there is also a back contact 109 on the opposite side of thesemiconductor substrate 105 from the intrinsic base 106, emitter 102,and collector 103.

Intrinsic base 106, emitter 102, and collector 103 may comprise one ofsilicon (Si), germanium (Ge), silicon germanium (SiGe), gallium arsenide(GaAs), indium arsenide (InAs), gallium nitride (GaN), silicon carbide(SiC), lithium fluoride (LiF), calcium fluoride (CaF₂), semiconductingcarbon nanotubes, and grapheme with appropriate dopants. Buriedinsulator layer 104 may comprise silicon oxide, or silicon nitride.Alternatively, buried insulator layer 104 may comprise silicon oxidecontaining conductive particles serving as charge traps. For example,buried insulator layer 104 may be silicon-rich oxide, which is siliconoxide with excess silicon. The excess silicon is known to function ascharge traps. Semiconductor substrate 105 may comprise one of silicon(Si), germanium (Ge), silicon germanium (SiGe), gallium arsenide (GaAs),indium arsenide (InAs), gallium nitride (GaN), silicon carbide (SiC),lithium fluoride (LiF), and calcium fluoride (CaF₂). Spacer material 108may comprise silicon oxide and/or silicon nitride. Optional back contact109 is electrically connected to semiconductor substrate 105.

The semiconductor layer (emitter 102, intrinsic base 106, and collector103) may be 50 nm thick, and the buried insulator layer 104 may be 140nm thick. Alternatively, the thickness of the semiconductor layer may bebetween about 20 and about 500 nm, and the thickness of the buriedinsulator layer may be between about 50 and about 500 nm. The thicknessof the semiconductor substrate 105 may be less than about 800 microns.

To operate the lateral SOI BJT, current is injected from the n+-emitter102, across the p-base barrier, intrinsic base 106, and reaches then+-collector 103. The base/emitter junction voltage is forward biased(V_(BE)>0) and the base/collector junction voltage is zero or reversebiased (V_(BC)≦0). If a positive back gate voltage (V_(X)>0) is appliedto semiconductor substrate 105 at optional back contact 109, depletionwill occur in intrinsic base 106 near buried oxide layer 104. Thedepletion lowers the barrier to current flow through intrinsic base 106,and leads to an increase in the collector current (I_(C)) at collector103. The base current (I_(B)) also decreases because of the increase inelectron concentration on the emitter region 102 near buried oxide layer104. The net result is the increase in collector current (and currentgain (I_(C)/I_(B))) in the lateral SOI BJT.

This result is illustrated in FIGS. 2 and 3, which are simulated dataplots of collector current (I_(C)) against base/emitter junction voltage(V_(BE)), and base current (I_(B)) against base/emitter junction voltage(V_(BE)), respectively, for several positive values of the back gatevoltage (V_(X)). Both FIGS. 2 and 3 show the effect of increasing thepositive back gate voltage (V_(X)). In FIG. 2, the collector current(I_(C)) increases as the back gate voltage (V_(X)) increases, asindicated by the arrow in the figure. In FIG. 3, the base current(I_(B)) either does not change or decreases, as indicated by the arrowin the figure, as the back gate voltage (V_(X)) increases. In any event,the collector current (I_(C)) and current gain (I_(C)/I_(B)) bothincrease as the positive back gate voltage (V_(X)) increases. In FIGS. 2and 3, T_(SOI) refers to the thickness of the semiconductor layer(emitter 102, base region 106, and collector 103) and T_(BOX) refers tothe thickness of the buried insulator layer 104, where BOX stands forburied oxide.

Positive charges generated by ionizing radiation and trapped in theburied oxide layer 104 have an effect equivalent to that of applying apositive back gate voltage (V_(X)>0) to semiconductor substrate 105 atoptional back contact 109. In this regard, FIG. 4 is a cross-sectionalview of the embodiment of the lateral SOI BJT radiation dosimeter 100shown in FIG. 1 after exposure to ionizing radiation. The radiation mayinclude, but is not limited to, high-energy ionizing radiation, protonbeam, X-ray, photons, gamma ray, or neutron beam radiation. Ionizingradiation causes electron-hole pairs to be created in buried insulatorlayer 104, which causes positive charges 202 to build up and be retainedin buried insulator layer 104. The buried insulator layer 104 maycomprise a plurality of charge traps in which positive charge 202 istrapped; the number of charge traps per cm³ of buried insulator materialmay be between about 1E17 and 1E18. The amount of positive charge 202 isindicative of the amount of radiation to which the radiation dosimeter100 has been exposed. In other words, the same increase in collectorcurrent (I_(C)) (and current gain I_(C)/I_(B)) will be observed in thelateral SOI BJT as may be observed when a positive back gate voltage(V_(X)>0) is applied to semiconductor substrate 105 at optional backcontact 109 under the biasing conditions described above. Whenappropriately calibrated, the collector current (and current gain) maybe used to record the radiation dose. Of course, a positive back gatevoltage (V_(X)>0) may also be applied to semiconductor substrate 105 atoptional back contact 109 to increase the trapping of positive charges(hence, larger collector current and current gain) in situations wherethe absorbed dose is low and difficult to detect. For this reason, theback contact 109 has been described as being optional.

For traditional applications in analog (as in amplifier circuits) anddigital (as in logic circuits) applications, high bipolar collectorcurrent (and current gain) and relatively high collector currents arerequired. These requirements call for designs having a narrow basewidth. The narrow base implies that the base region be heavily doped,that is, to provide about 1E19 charge carriers per cm³ for a base widthof about 15 nm. In addition, a thin silicon layer (that is, emitter 102,intrinsic base 106, and collector 103) is preferred for a low baseresistance.

For sensor applications, such as for radiation dosimeters, the mostcritical property is the shift of the bipolar turn-on voltage as afunction of the charge in the insulator induced by the externalenvironment. This sensitivity does not depend on the base width of thebipolar junction transistor, so the base width can be designed to bemuch wider (greater than 15 nm) to increase the sensing area. Thecorrespondingly much lighter doped base region (less than 1.0×10¹⁹charge carriers per cm³) further enhances the sensitivity of thecollector current to the charge in the insulator. This makes bipolarjunction transistors for radiation dosimetry applications fundamentallydifferent from the bipolar junction transistors used in traditionalcircuit applications. Furthermore, base resistance is not as importantfor sensor applications and a thick silicon layer is preferred tomaintain a steep turn-on behavior unlike the traditional application.

FIG. 5 illustrates an embodiment of a method 500 for radiationmonitoring using a lateral SOI BJT radiation dosimeter. FIG. 5 isdiscussed with reference to FIG. 4. In block 502, a lateral SOI BJTradiation dosimeter is provided. Several options for using the radiationdosimeter are presented in blocks 512, 514, 516, and 518. In block 512,the radiation dosimeter is placed inside a body to determine a dose ofradiation received by the body. In block 514, the radiation dosimeter isplaced proximate to a body to determine a dose of radiation received bythe body during medical imaging. In block 516, at least one filter layeris placed between the radiation dosimeter and a source of the radiation.And, in block 518, the radiation dosimeter is incorporated into one ofan automobile, a building, an air filter, a portable electronic device,such as a computer, cell phone, music player, PDA or GPS, a passport,credit card, or driver license. Other options will readily occur tothose of ordinary skill in the art.

In block 504 the radiation dosimeter 100 is exposed to ionizingradiation 200, causing electron-hole pairs to be created in buriedinsulator layer 104. The amount of positive charge 202 is indicative ofthe amount of radiation to which the radiation dosimeter 100 has beenexposed. In block 504, the collector current (I_(C)) and current gain(I_(C)/I_(B)) of the radiation dosimeter change based on the amount ofradiation to which the radiation dosimeter has been exposed, due topositive charge 202 built up in buried insulator layer 104. In block506, the increase in the collector current (I_(C)), that is, thedifference between the value of the collector current (I_(C)) before andafter exposure to radiation, is obtained. Alternatively, the increase inthe current gain (I_(C)/I_(B)) may be obtained. In block 508, the amountof radiation exposure is determined based on the change in the collectorcurrent (I_(C)) or on the change in the current gain (I_(C)/I_(B)).

In summary, the present lateral SOI BJT radiation dosimeter has theadvantage of having the I_(C)−V_(BE) characteristics of an ideal 60mV/decade. The semiconductor layer (emitter 102, intrinsic base 106,collector 103) may be thicker than 40 nm. The V_(T) (threshold voltage)shift from buried insulator layer charge can be less sensitive to SOIthickness. Finally, the buried insulator layer 104 shows good chargeretention.

A single semiconductor substrate 105 may hold a plurality of lateral SOIBJT radiation dosimeters, each SOI BJT radiation dosimeter comprising aseparate extrinsic base 101, emitter 102, intrinsic base 106, collector103, buried insulator layer 104, semiconductor substrate 105, and,optionally, back contact 109. A plurality of SOI BJT radiationdosimeters may also be arranged in an array, including, but not limitedto, a linear array, a two-dimensional array, or a three-dimensionalarray, in order to detect radiation doses in different areas and fromdifferent directions. The different directions may be orthogonal to oneanother. In some embodiments, a filter may be disposed between a SOI BJTradiation dosimeter and the source of the radiation 200 to prevent someof radiation 200 from passing through the device, or to make the devicemore or less sensitive to the type of incident radiation. Another typeof device may also be incorporated into the semiconductor substrate 105,including, but not limited to, a memory cell, a clock, a microprocessor,a DNA sensor, a biological sensor, a hazardous material sensor, aglucose sensor, a red blood cell sensor, or a camera.

The technical effects and benefits of exemplary embodiments include arelatively small, inexpensive radiation dosimeter that may be used todetermine long-term or real-time radiation dosage information.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an”, and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising”, when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A radiation dosimeter comprising: a semiconductorsubstrate; a buried insulator layer disposed on said semiconductorsubstrate, said buried insulator layer comprising a plurality of chargetraps; and a semiconductor layer disposed on said buried insulatorlayer, said semiconductor layer having an emitter, an intrinsic base,and a collector laterally arranged with respect to one another, wherein,when said radiation dosimeter is exposed to radiation, positive chargesare trapped in said plurality of charge traps in said buried insulatorlayer, the amount of positive charge trapped being used to determine theamount of radiation exposure.
 2. The radiation dosimeter as claimed inclaim 1, further comprising a back contact electrically connected tosaid semiconductor substrate, said back contact being configured toreceive a back gate voltage, said back gate voltage providing a positivebias across said buried insulator layer to enhance the positive chargetrapped in the charge traps in said buried insulator layer in responseto radiation exposure by said radiation dosimeter.
 3. The radiationdosimeter of claim 1, wherein a number of charge traps per cm³ of theburied insulator layer is between 1E17 to 1E18.
 4. The radiationdosimeter as claimed in claim 1, wherein a thickness of saidsemiconductor substrate less than about 800 microns.
 5. The radiationdosimeter as claimed in claim 1, wherein said semiconductor substratecomprises one of silicon (Si), germanium (Ge), silicon germanium (SiGe),gallium arsenide (GaAs), indium arsenide (InAs), gallium nitride (GaN),silicon carbide (SiC), lithium fluoride (LiF), and calcium fluoride(CaF₂).
 6. The radiation dosimeter as claimed in claim 1, wherein athickness of said buried insulator layer is between about 50 and about500 nm.
 7. The radiation dosimeter as claimed in claim 6, wherein athickness of said buried insulator layer is 140 nm.
 8. The radiationdosimeter as claimed in claim 1, wherein said buried insulator layercomprises silicon oxide containing conductive particles serving ascharge traps.
 9. The radiation dosimeter as claimed in claim 1, whereinsaid buried insulator layer comprises silicon-rich oxide.
 10. Theradiation dosimeter as claimed in claim 1, wherein said buried insulatorlayer comprises one of silicon oxide (SiO₂) or silicon nitride (Si₃N₄).11. The radiation dosimeter as claimed in claim 1, wherein the thicknessof said semiconductor layer is between about 20 and about 500 nm. 12.The radiation dosimeter as claimed in claim 10, wherein the thickness ofsaid semiconductor layer is 50 nm.
 13. The radiation dosimeter asclaimed in claim 1, wherein said semiconductor layer comprises one ofsilicon (Si), germanium (Ge), silicon germanium (SiGe), gallium arsenide(GaAs), indium arsenide (InAs), gallium nitride (GaN), silicon carbide(SiC), lithium fluoride (LiF), calcium fluoride (CaF₂), semiconductingcarbon nanotubes, and graphene.
 14. The radiation dosimeter as claimedin claim 1, wherein said intrinsic base of said semiconductor layer hasa width greater than 15 nm.
 15. The radiation dosimeter as claimed inclaim 1, wherein said intrinsic base of said semiconductor layer hasfewer than 1E19 charge carriers per cm³.
 16. The radiation dosimeter asclaimed in claim 1, further comprising an extrinsic base in electricalcontact with said intrinsic base.
 17. The radiation dosimeter as claimedin claim 16, further comprising spacer material around said extrinsicbase.
 18. The radiation dosimeter as claimed in claim 17, wherein saidspacer material comprises one of silicon oxide (SiO₂) or silicon nitride(Si₃N₄).
 19. The radiation dosimeter as claimed in claim 1, furthercomprising at least one additional semiconductor layer disposed on saidburied insulator layer, said additional semiconductor layer having anemitter, an intrinsic base, and a collector laterally arranged withrespect to one another to form a second radiation dosimeter.